Organic transistor, manufacturing method of semiconductor device and organic transistor

ABSTRACT

It is an object to form a high quality gate insulating film which is dense and has a strong insulation resistance property, and to propose a high reliable organic transistor in which a tunnel leakage current is little. One mode of the organic transistor of the present invention has a step of forming the gate insulating film by forming the conductive layer which becomes the gate electrode activating oxygen (or gas including oxygen) or nitrogen (or gas including nitrogen) or the like using dense plasma in which density of electron is 10 11  cm −3  or more, and electron temperature is a range of 0.2 eV to 2.0 eV with plasma activation, and reacting directly with a portion of the conductive layer which becomes the gate electrode to be insulated.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an organic transistor and amanufacturing method of an organic transistor having an organicsemiconductor layer and a semiconductor device having an organictransistor.

2. Description of the Related Art

A field-effect transistor controls an electric conductivity of asemiconductor layer which is provided between a source and a drainelectrodes with a voltage applied to a gate electrode, and afield-effect transistor is a representative one of unipolar elementsusing carrier transport of either holes or electrons.

Since various kinds of switching elements and amplifying elements can beformed depending on a combination of such field-effect transistors,these field-effect transistors are applied in various fields. Forexample, a switching element of a pixel in an active matrix display orthe like can be given as the application.

An inorganic semiconductor material represented by silicon has been usedbroadly as a semiconductor material using for the field-effecttransistor. Since a high-temperature treatment is needed for forming afilm of an inorganic semiconductor material as a semiconductor layer, itis difficult to use plastic or a film as a substrate.

In contrast with this, since a film can be formed at even lowtemperature relatively, a field-effect transistor can be manufactured inprinciple over a substrate of which heat endurance is low such as aplastic substrate as well as a glass substrate when an organicsemiconductor material is used as a semiconductor layer.

An organic transistor which is formed by forming a film thesemiconductor layer including the organic semiconductor material in alow temperature process is disclosed in Patent Document 1. Note that agate insulating film of the organic transistor written in PatentDocument 1 is formed by a plasma CVD method.

[Patent Document 1]

Japanese Patent Laid-Open No. 2000-174277

With a minituarization of the transistor, it is necessary to make thegate insulating film thinner at the same time as shortening a channellength. However, when the gate insulating film is made thinner, a tunnelleakage current is high, and there is a concern about degradation ofreliability. Therefore, the gate insulating film which is a higherresistant property is needed.

BRIEF SUMMARY OF THE INVENTION

It is an object of the present invention to provide a lower tunnelleakage current organic transistor by forming a dence and highlyinsulating resistance property gate insulating film than the gateinsulating film formed by using a conventional CVD method.

The present invention takes a method of hereinafter to accomplish theabove object.

One mode of an organic transistor of the present invention is atransistor by obtained by forming a conductive layer which becomes agate electrode; activating oxygen (or a gas including oxygen) ornitrogen (or a gas including nitrogen) or the like using dense plasma ofwhich density of electrons is 10¹¹ cm⁻³ or more and electron temperatureis a range of 0.2 eV to 2.0 eV by plasma activation; and forming thegate insulating film by reacting directly and insulating with a part ofthe conductive layer which becomes the gate electrode.

One mode of an organic transistor of the present invention is having agate electrode, a gate insulating film, a semiconductor layer includingan organic semiconductor material, in which source and drain electrodes,and the source and drain electrodes have a composite layer of an organiccompound and a metal oxide and a conductive layer, and the gateinsulating film is formed by performing dense plasma treatment to theconductive layer which becomes a gate electrode.

Another mode of an organic transistor of the present invention is havinga gate electrode, a gate insulating film formed in contact with and overthe gate electrode, a semiconductor layer including an organicsemiconductor material formed over the gate insulating film, and sourceand drain electrodes over a semiconductor layer, in which the source anddrain electrodes have a composite layer of an organic compound and ametal oxide and the conductive layer, the gate insulating film is formedby performing dense plasma treatment to the conductive layer whichbecomes the gate electrode.

Another mode of an organic transistor of the present invention is havinga gate electrode, a gate insulating film formed over the gate electrode,a semiconductor layer including an organic semiconductor material formedover the gate insulating film, and a source and a drain electrodes overthe semiconductor layer, in which the source and drain electrodes have acomposite layer of an organic compound and a metal oxide and theconductive layer, and the gate insulating film is a film which issubjected to dense plasma treatment.

One of manufacturing methods of the organic transistor of the presentinvention includes a steps of forming a first conductive layer over asubstrate, insulating a surface of the first conductive layer by denseplasma treatment, forming a semiconductor layer including an organicsemiconductor material over the first insulated conductive film, forminga composite layer in which an organic compound and a metal oxide aremixed over the semiconductor layer and forming a second conductive layerover the composite layer. At this time, the composite layer and thesecond conductive layer are the source and drain electrodes, and aportion having a conductive property of the first conductive layer is agate electrode, and a portion having an insulating property of the firstconductive layer is a gate insulating film.

Another manufacturing method of an organic transistor of the presentinvention is forming a gate electrode over a substrate, forming a gateinsulating film over a gate electrode, forming the semiconductor layerincluding an organic semiconductor material over the gate insulatingfilm, forming a composite layer in which an organic compound and a metaloxide are mixed over the semiconductor layer, forming a layer over thecomposite layer. At this time, the composite layer and the conductivelayer is the source and drain electrodes, and dense plasma treatment isperformed to the gate insulating film.

Dense plasma treatment uses plasma in which density of electrons is 10¹¹cm⁻³ or more and electron temperature is a range of 0.2 eV to 2.0 eV(more preferably, a range of 0.5 eV to 1.5 eV). Note that “dense plasma”in this specification may be also called “high density plasma”.

The gate electrode may be any one of tantalum, niobium, aluminum,molybdenum, tungsten, titanium, copper, chromium, nickel, cobalt, andmagnesium.

The gate insulating film may have dielectric constant of 8 or more.

The organic compound which forms the composite layer may have anaromatic amine skeleton.

The metal oxide which forms the composite layer may be one metal oxideor plural metal oxides selected from titanium, vanadium, chromium,zirconium, niobium, molybdenum, hafnium, tantalum, tungsten, andrhenium.

One mode of the semiconductor device of the present invention is that acircuit is formed by using an organic transistor in which an insulatingfilm which is formed by using dense plasma in which density of electronsis 10¹¹ cm⁻³ or more, and electron temperature is a range of 0.2 eV to2.0 eV (more preferably, a range of 0.5 eV to 1.5 eV) and asemiconductor layer including an organic semiconductor material arecontacted.

Since a gate insulating film which is formed by using dense plasma canbe a high quality film which is less damaged by plasma and has almost nodefects, the gate insulating film can reduce a tunnel leakage current.Therefore, a highly reliable organic transistor can be obtained. Inaddition, when the conductive layer which becomes a gate electrode isinsulated to be the gate insulating film, high integration is enabled.

BRIEF DESCRIPTION OF THE DRAWING

FIGS. 1A and 1B are diagrams explaining a configuration of an organictransistor of the present invention;

FIGS. 2A to 2E are diagrams explaining a manufacturing method of anorganic transistor of the present invention;

FIGS. 3A and 3B are diagrams explaining configurations of organictransistors of the present invention;

FIGS. 4A and 4B are diagrams explaining configurations of organictransistors of the present invention;

FIGS. 5A and 5B are diagrams explaining configurations of organictransistors of the present invention;

FIG. 6 is a schematic view of a dense plasma treatment device;

FIG. 7 is a top view of a liquid crystal display device using thepresent invention;

FIG. 8 is a cross-sectional view of a liquid crystal display deviceusing the present invention;

FIG. 9 is a cross-sectional view of a light-emission display deviceusing the present invention; and

FIGS. 10A to 10D are diagrams of electronics devices using the presentinvention.

DETAILED DESCRIPTION OF THE INVENTION

Embodiment Modes of the present invention are hereinafter described indetail with reference to the drawings. However, the present invention isnot limited to the following description, and it is to be understood'bythose skilled in the art that the mode and detail of the presentinvention can be changed variously within the scope of the presentinvention. Therefore, the present invention should not be construed asbeing limited to the description of the following Embodiment Modes andEmbodiments. Further, in the structures of the present inventionhereinafter described, the same parts are denoted with the samereference numerals throughout the drawings.

Embodiment Mode 1

A configuration of an organic transistor structure of the presentinvention is shown in FIGS. 1A and 1B. Note that reference numeral 10 isa substrate, 11 is a gate electrode, 12 is a gate insulating film, 13 isa semiconductor layer including an organic semiconductor material, 14 isa composite layer having an organic compound and a metal oxide, 15 is aconductive layer, and a source and a drain electrodes 16 a and 16 b havea composite layer 14 and a conductive layer 15. Arrangement of eachlayer and electrode can be selected suitably depending of the use of theelement. In addition, the composite layer 14 is formed to be in contactwith the semiconductor layer 13 in FIGS. 1A and 1B; however, withoutbeing limited to this, the composite layer may be included in a portionof a source electrode and/or a drain electrode.

A structure of FIG. 1A is explained along manufacturing methods of FIGS.2A to 2E. As the substrate 10, an insulating property substrate such asa glass substrate, a quartz substrate, a crystalline glass, a ceramicsubstrate, a stainless steel substrate, a plastic substrate (polyimide,acryl, polyethylene terephthalate, polycarbonate, polyarylate,polyethersulfone or the like) or the like can be used. In addition,these substrates may be used after polishing by CMP or the like asnecessary.

A conductive layer 17 which becomes a gate electrode is formed over thesubstrate 10 (see FIG. 2A). A metal having an insulating property bynitrirling and/or oxygenating may be used as the material of aconductive layer 17. Specifically, tantalum, niobium, aluminum,molybdenum, copper, or titanium is preferred. In addition, tungsten,chromium, nickel, cobalt, magnesium and the like can be given. Amanufacturing method of the conductive layer 17 is not particularlylimited, and after forming a film by a sputtering method or anevaporation method or the like, the film may be manufactured to have adesirable shape by an etching method or the like. In addition, the filmmay be formed by using a droplet including a conductive substance by anink-jet printing method or the like.

Next, the gate insulating film 12 including a nitride, an oxide, or anoxynitride of the metal is formed by nitriding and/or oxygenating theconductive layer 17 using dense plasma (see FIG. 2B). Dense plasma isproduced by using a micro wave of high frequency, for example, using2.45 GHz. Such dense plasma is used, and oxygen (or a gas includingoxygen), nitrogen (or a gas including nitrogen) or the like is activatedby plasma activation, and these are reacted with a material of the gateelectrode directly to insulate the conductive layer 17.

Dense plasma of which density of electron is 10¹¹ cm ⁻³ or more, andelectron temperature is a range of 0.2 eV to 2.0 eV (more preferably, arange of 0.5 eV to 1.5 eV) is used. Such dense plasma which ischaracterized low electron temperature can form a film which is lessdamaged by plasma and has almost no defects than conventional plasmatreatment since energy of motion of active species is low. In addition,this insulating film is denser than an insulating film which is formedby using an anodic oxidation method. The conductive layer 17 can performas the gate electrode 11 except for the gate insulating film 12insulated by using dense plasma of the conductive layer 17.

For example, dense plasma treatment is performed using a device of FIG.6. Reference numeral 61 is a dielectric waveguide, 62 is a slot antennahaving plural slots, 63 is a dielectric substrate which is made ofquartz or aluminum oxide, and 64 is a stage for installing a substrate.The stage 64 has a heater. A micro wave is transmitted from 60, and agas which is supplied from a gas supply port 65 in a plasma generatingregion 66 is activated. A position and a length of the slot in the slotantenna 62 are selected suitably depending on a wave length of the microwave transmitted from 60.

By using such a device, plasma with uniformity, highly density, and lowelectron temperature can be activated, and low temperature treatment(substrate temperature 400° C. or less) can be achieved. Note, thatplastics, which are thought to have low heat resistance generally, canbe used as a substrate.

Note that gas in which oxygen (or gas including oxygen) or nitrogen (orgas including nitrogen) is mixed into an inert gas such as argon,krypton, helium, or xenon is used as a gas to be supplied. Therefore,these inert elements are mixed into the gate insulating film formed bydense plasma oxidation or nitriding treatment. Hydrogen may be includedin a gas to be supplied.

Further, an activated gas which is more uniform can be supplied to aprocessing object by providing a shower plate in a device inside 67. Indescription below, dense plasma treatment in manufacturing of the gateinsulating film is performed by using plasma having the abovecharacteristics.

Next, a semiconductor layer 13 covering a gate insulating film 12 isformed (see FIG. 2C). The organic semiconductor material forming thesemiconductor layer 13 has a carrier transporting property, and if theorganic material can have modulation of carrier density by electricfield-effect, low molecular and high molecular materials can be used,and the kind is not limited particularly. A polycyclic aromaticcompound, a conjugate double bond compound, a metallophthalocyaninecomplex, a charge transfer complex, a condensed ring tetracarboxylicdiimide type, an oligothiophene type, a fullerene type, a carbonnanotube or the like can be given. For example, polypyrrole,polythiophene, poly(3-alkylthiophene), polythienylenevinylene,poly(p-phenylenevinylene), polyaniline, polyazulene, polypyrene,polycarbazole, polyselenophene, polyfuran, poly (p-phenylene),polyindole, polypyridazine, naphthacene, hexacene, heptacene, pyrene,chrysene, perylene, coronene, terrylene, ovalene, quaterrylene,triphenodioxazine, triphenodiriazine, hexacene-6, 15-quinone,polyvinylcarbazole, polyphenylenesulfide, polyvinylenesulfide,polyvinylpyridine, naphthalenetetracarboxylic diimide,anthracenetetracarboxylic diimide, C60, C70, C76, C78, C84, andderivatives thereof can be used. In addition, as concrete examplesthereof, there is tetracene, pentacene, sexithiophene (6T), copperphthalocyanine, bis-(1,2,5-thiadiazolo)-p-quinobis(1,3-dithiol),ruburene, poly (2,5-thienylenevinylene) (PTV), poly(3-hexylthiophene-2,5-diyl) (P3HT), or poly(9,9′-dioctyl-fluorene-co-bithiophene) (F8T2) which is generallycategorized in a P-type semiconductor, and7,7,8,8,-tetracyanoquinodimethane (TCNQ),3,4,9,10-perylenetetracarboxylicdianhydride (PTCDA),1,4,5,8-naphthalenetetracarboxylicdianhydride (NTCDA),11,11,12,12,-tetracyano-1,4-naphthaquinodimethane (TCNNQ),N,N′-dioctyl-3,4,9,10-p erylenetetracarboxylicdiimide (PTCDI-C8H),copper16phthalocyaninefluoride (F₁₆CuPc), or3′,4′-dibutyl-5,5″-bis(dicyanomethylene)-5,5″-dihydro-2,2′:5′,2″-terthiophen) (DCMT), or the like which is generally categorized in anN-type semiconductor. Note that in the organic semiconductor, theproperty of the P-type of the N-type is not an inherent property, and isrelied in relation with an electrode injecting carriers, or intensity ofelectrical field when carrier injection is performed. There is atendency that it is easy to become the P-type or the N-type; however,the organic semiconductor can be used as a P-type semiconductor and anN-type semiconductor. In this embodiment mode, a P-type semiconductor ismore preferred.

These organic semiconductor materials can be formed by a method such asan evaporation method, spin coating method, or a droplet dischargingmethod.

Next, a composite layer 14 is formed over the semiconductor layer 13(see FIG. 2D). A kind of the organic compound used for the compositelayer 14 of the present invention is not limited particularly, forexample, 4,4′-bis[N-(1-naphtyl)-N-phenylamino]biphenyl (NPB),N,N′-diphenyl-N,N′-bis(3-methylphenyl)-1,1′-biphenyl-4,4′-diamine (TPD),4,4-bis(N-{4-(N,N-di-m-tolylamino)phenyl}-N-phenylamino)biphenyl(DNTPD), 4,4′,4″-tris[N-(3-methylphenyl)-N-phenylamino]triphenylamine(m-MTDATA), or 4,4′,4″-tris[N-(1-naphthyl)-N-phenylamino]triphenylamine(1-TNATA) which has an aromatic amine skeleton is preferable. Inaddition, an N-arylcarbazole derivative such as N-(2-naphthyl)carbazole(NCz), 4,4′-di(N-carbazolyl)biphenyl (CBP), or anthracene, or anaromatic carbon hydride such as 9,10-diphenylanthracene (DPA) or thelike can be used. In addition, a material which can be used as thesemiconductor 13 can be used. In this case, adhesion and chemicalstability of boundary face of the semiconductor layer 13 and thecomposite layer 14 are improved. Then, an advantage that a manufacturingprocess becomes easily can be given.

A metal oxide used for the composite layer 14 of the present inventionis not limited particularly. An oxide of titanium, vanadium, chromium,zirconium, niobium, molybdenum, hafnium, tantalum, tungsten, or rheniumis preferable. The composite layer including a metal oxide at the rangeof 5 wt % to 80 wt %, preferably, the range of 10 wt % to 50 wt % ispreferable.

The organic compound such as 7,7,8,8-tetracyanoquinodimethane (TCNQ),2,3,5,6-tetrafluoro7,7,8,8-tetracyanoquinodimethane (F₄-TCNQ) whichrepresents an electron-accepting property may be used in substitutionfor a metal oxide.

The composite layer 14 may be formed by a co-evaporation usingresistance heating, by a co-evaporation using resistance heating and anelectron gun evaporation

(EB evaporation), or by simultaneously by a sputtering method andresistance heating or the like. In addition, the film formation may beperformed by using a wet method such as a sol-gel method.

Since the electric conductivity of the composite layer 14 is as high asabout 10⁻⁵ [S/cm], and even when the film thickness is changed fromseveral nm to several hundred nm, the change a value of resistance of atransistor is small, the film thickness of the composite layer can becontrolled suitably from several nm to several hundred nm or moredepending on an application or shape of an element which is formed.

Next, a conductive layer 15 is formed (see FIG. 2E). A material used forthe conductive layer 15 is not limited particularly. A metal such asgold, platinum, aluminum, tungsten, titanium, copper, molybdenum,tantalum, niobium, chromium, nickel, cobalt, magnesium, an alloyincluding them, and a conductive high molecular compound such aspolyaniline, polypyrrole, polythiophen, polyacethylene, polydiacetylenecan be given. Generally, a metal is used often for the conductive layer15 to be used for source and drain electrodes 16 a and 16 b.

A forming method is not limited particularly as long as thesemiconductor layer 13 does not decompose. After forming the film by asputtering method or an evaporation method or the like, the conductivelayer 15 may be processed in the desirable shape by an etching method orthe like and manufactured. In addition, the conductive layer 15 may beformed by an ink-jet printing method using a droplet including aconductive substance or the like.

In the organic transistor manufactured according to the above method, anenergy barrier between the semiconductor layer 13 and the source anddrain electrodes 16 a and 16 b is reduced by using the source and drainelectrodes 16 a and 16 b which have a structure in which the compositelayer 14 is interposed between the semiconductor layer 13 and theconductive layer 15, and carrier injection from one of the electrode ofthe source and drain electrodes to the semiconductor layer and carrierdischarging from the semiconductor layer to the other electrode becomeeasily. Consequently, a material of the conductive layer 15 does nothave to select the material in which the energy barrier with thesemiconductor 13 is low, and can be selected without a constraint of awork function.

In addition, the composite layer 14 is superior in a carrier injectionproperty and chemically stable, and adhesion with the semiconductor 13is better than the conductive layer 15. The source and drain electrodes16 a and 16 b of the present invention can be used also as a wiring.

Since the gate insulating film 12 formed by using dense plasma has fewplasma damage and defects, a tunnel leakage current can be reduced.Since asperity of the surface is small, carrier mobility can be high.Further, it makes orientation of the organic semiconductor materialwhich makes the semiconductor layer 13 formed over the gate insulatingfilm easy. In addition, high dielectric constant gate insulating filmcan be formed by selecting a material such as Ta or Al to the gateelectrode which becomes high dielectric constant by a nitridingtreatment or an oxidation treatment.

Therefore, even if the gate insulating film is made a thinner, anequivalent oxide film thickness (EOT: Equivalent Oxide Thickness) can begained, and high speed operation can be performed while preventing atunnel leakage current. Further, since a width of the gate electrode canbe narrowed and the gate electrode can be made thinner by reacting thegate electrode directly, a channel-length can be shortened. Therefore,high integration is enabled.

An organic insulating material such as polyimide, polyamic acid, orpolyvinylphenyl may be formed a film in contact with a under surface ofthe semiconductor layer 13. With such a structure, orientation of theorganic semiconductor material is more improved, and adhesion of thegate insulating film 12 and the semiconductor layer 13 can be moreimproved.

In addition, a structure in which the source and drain electrodes 16 aand 16 b are provided over the semiconductor layer 13 like a structureof FIG. 1A (hereinafter referred to as a top contact type structure) isexemplified; however, in the present invention, a structure in which thesource and drain electrodes are provided under the organic semiconductorlayer like a structure of FIG. 1B (hereinafter referred to as a bottomcontact structure) may be used.

In a case of the bottom gate type, when the top contact type structureis adopted, there is an advantage in which carrier mobility is high.Meanwhile, in a case of using the bottom contact type structure, aprocess such as photolithography can be used easily formicrofabricaction of the source and drain wirings. Therefore, thestructure of the organic transistor may be selected in accordance withdrawback and advantage.

As described above, the high reliable organic transistor can beprovided.

Embodiment Mode 2

The organic transistor as shown in embodiment mode 1 uses the insulatingfilm formed by performing dense plasma treatment to the conductive layerwhich becomes the gate electrode as the gate insulating film; however,in this embodiment mode, a gate insulating film is formed by nitridingor oxygenizing with dense plasma additionally to the insulating filmformed previously like FIGS. 3A and 3B. Since, elements except the gateinsulating film and the gate electrode are similar to those ofEmbodiment Mode 1, they are denoted by the same reference numerals andthe description thereof is omitted.

A gate electrode 31 is formed over a substrate 10. A material using forthe gate electrode 31 is not limited particularly. For example, a metalsuch as gold, platinum, aluminum, tungsten, titanium, copper,molybdenum, tantalum, niobium, chromium, nickel, cobalt, magnesium andan alloy including them, a conductive high molecular compound such aspolyaniline, polypyrrole, polythiophene, polyacetylene, polydiacetylene,and polysilicon doped with an impurity can be given. A method formanufacturing the gate electrode 31 is not limited particularly, afterforming by a sputtering method, an evaporation method or the like, itmay be processed into a desired shape and manufactured by an etchingmethod or the like. In addition, an ink-jetting method or the like maybe used by using a droplet including a conducting substance.

Next, an insulating film 32 covering the gate electrode 31 is formed. Aninorganic material such as silicon oxide, silicon nitride or siliconoxynitride is used for the insulating film 32. These insulating films 32can be formed by an application method such as a dipping method, a spincoating method, a droplet discharging method, and a CVD method, asputtering method.

The gate insulating film is formed by performing nitriding treatment oroxidation treatment using dense plasma to this insulating film 32. Forexample, silicon oxynitride is formed by performing dense plasmanitriding treatment to the insulating film 32 of silicon oxide, orperforming dense plasma oxidation treatment to the insulating film 32 ofsilicon nitride. In addition, the gate insulating film which has astacked structure of silicon oxide or silicon nitride and siliconoxynitride may be formed. The number of the gate insulating films to bestacked is not limited especially. A silicon nitride film includinghighly concentration nitrogen can be obtained by performing dense plasmanitriding to silicon nitride.

Dense plasma is produced by using high frequency micro wave e.g, 2.45GHz. Activation oxygen (or gas including oxygen) or nitrogen (or gasincluding nitrogen) is activated by plasma activation by using suchdense plasma, and is reacted with the insulating film. Since energy ofmotion of active species is low, dense plasma in which low electrontemperature is characteristic can form few plasma damage andless-defective film compared with conventional plasma treatment. Notethat an inert gas used for a gas to be supplied which is shown inEmbodiment Mode 1 is mixed.

A semiconductor layer 13 is formed over the gate insulating film. Next,the source and drain electrodes 16 a and 16 b are formed.

In the organic transistor having such a structure, an energy barrierbetween the semiconductor layer 13 and the source and drain electrodes16 a and 16 b is reduced by using the source and drain electrodes 16 aand 16 b which have a structure in which the composite layer 14 isinterposed between the semiconductor layer 13 and the conductive layer15, and carrier injection from one of the electrode of the source anddrain electrodes to the semiconductor layer and carrier discharging fromthe semiconductor layer to the other electrode become easily.Consequently, a material of the conductive layer 15 does not have toselect the material in which the energy barrier with the semiconductor13 is low, and can be selected without a constraint of a work function.

In addition, the composite layer 14 is superior in a carrier injectionproperty and chemically stable, and adhesion with the semiconductor 13is better than the conductive layer 15. The source and drain electrodes16 a and 16 b of the present invention can be used also as a wiring.

Since the gate insulating film formed by using dense plasma is fewplasma damage and defects, a tunnel leakage current can be reduced.Since asperity of the surface is small, carrier mobility can be high.Further, it makes orientation of the organic semiconductor materialwhich makes the semiconductor layer 13 formed over the gate insulatingfilm easy.

The top contact type structure like FIG. 3A is exemplified; however, inthe present invention, the bottom contact type structure like FIG. 3Bmay be used.

In a case of the bottom gate type, when the top contact type structureis adopted, there is an advantage in which carrier mobility is higher.Meanwhile, in a case of using the bottom contact type structure, aprocess such as photolithography can be used easily for providingmicrofabricaction of the source and drain electrode wirings.

Therefore, the structure of the organic transistor may be selected inaccordance with drawback and advantage.

In addition, the conductive layer which becomes the gate electrode isinsulated by using dense plasma like embodiment mode 1, and the obtainedinsulating film may be used as a portion of the gate insulating film.Note that a material of the gate electrode which can be used at thattime is the material of the conductive layer 17 denoted in embodimentmode 1. In this case, since a width of the gate electrode can benarrowed and the .gate electrode can be made thinner by reacting thegate electrode directly, a channel-length can be shortened. Therefore,high integration is enabled.

As described above, the high reliable organic transistor can beprovided.

Embodiment Mode 3

In this embodiment mode, an organic transistor having a structure whichis different from the structure denoted in Embodiment Modes 1 and 2 isexplained using FIG. 4A and 4B. The organic transistor of the embodimentmodes 1 and 2 is the bottom gate type transistor; however, thetransistor of this embodiment mode is a top gate type transistor.Portions similar to those of Embodiment Mode 1 are denoted by the commonreference numerals and the description thereof is omitted.

A structure of FIG. 4A is explained. The semiconductor layer 13 isformed over the substrate 10. Further, the source and drain electrodes16 a and 16 b having the composite layer 14 and the conductive layer 15are formed over the semiconductor layer 13.

Next, an insulating film 42 covering the semiconductor layer 13 and thesource and drain electrodes 16 a and 16 b is formed. An inorganicmaterial such as silicon oxide, silicon nitride or silicon oxynitride isused for the insulating film 42. These insulating films 32 can be formedby an application method such as a dipping method, a spin coatingmethod, a droplet discharging method, and a CVD method, a sputteringmethod. Note that it is necessary to use such a condition or a methodthat the semiconductor layer 13 is not broken.

The gate insulating film is formed by performing nitriding treatment oroxidation treatment using dense plasma to this insulating film. Forexample, silicon oxynitride is formed by performing dense plasmanitriding treatment to the insulating film 42 of silicon oxide, orperforming dense plasma oxidation treatment to the insulating film 42 ofsilicon nitride. In addition, the gate insulating film which has astacked structure of silicon oxide or silicon nitride and siliconoxynitride may be formed. The number of the gate insulating film to bestacked is not limited especially. A silicon nitride film includinghighly concentration nitrogen can be obtained by performing dense plasmanitriding to silicon nitride.

Dense plasma is produced by using high frequency micro wave e.g, 2.45GHz. Activation oxygen (or gas including oxygen) or nitrogen (or gasincluding nitrogen) is activated by plasma activated by using such highdense plasma, and is reacted with the insulating film. Since energy ofmotion of active species is low, dense plasma in which low electrontemperature is characteristic can form little plasma damage andless-defective film compared with conventional plasma treatment. Notethat an inert gas used for a gas to be supplied which is shown inEmbodiment Mode 1 is mixed.

Next, a gate electrode 41 is formed. A material using for the gateelectrode 41 is not limited particularly. For example, a metal such asgold, platinum, aluminum, tungsten, titanium, copper, molybdenum,tantalum, niobium, chromium, nickel, cobalt, magnesium and an alloyincluding them, a conductive high molecular compound such aspolyaniline, polypyrrole, polythiophene, polyacetylene, polydiacetylene,and polysilicon doped with an impurity can be given. A method formanufacturing the gate electrode 41 is not limited particularly, afterforming by a sputtering method, an evaporation method or the like, itmay be processed into a desired shape by an etching method or the like.In addition, an ink-jetting method or the like may be used by using adroplet including a conducting substance. Note that it is necessary touse such a condition or a method that the semiconductor layer is notbroken.

In the oaganic transistor having such a structure, an energy barrierbetween the semiconductor layer 13 and the source and drain electrodes16 a and 16 b is reduced by using the source and drain electrodes 16 aand 16 b which have a structure in which the composite layer 14 isinterposed between the semiconductor layer 13 and the conductive layer15, and carrier injection from one of the electrode of the source anddrain electrodes to the semiconductor layer and carrier discharging fromthe semiconductor layer to the other electrode become easily.Consequently, a material of the conductive layer 15 does not have toselect the material in which the energy barrier with the semiconductor13 is low, and can be selected without a constraint of a work function.

In addition, the composite layer 14 is superior in a carrier injectionproperty and chemically stable, and adhesion with the semiconductor 13is better than the conductive layer 15. The source and drain electrodes16 a and 16 b of the present invention can be used also as a wiring.

Since the gate insulating film formed by using dense plasma is fewplasma damage and defects, a tunnel leakage current can be reduced.

The top contact type structure like FIG. 4A is exemplified; however, inthe present invention, the bottom contact type structure like FIG. 4Bmay be used.

Furthermore, the heat resistance can be improved by performing nitridingtreatment to the gate electrode by using dense plasma. In the case ofinsulating the conductive layer which becomes the gate electrode, sincea width of the gate electrode can be narrowed and the gate electrode canbe made thinner, a channel-length can be shortened. Therefore, highintegration is enabled.

As described above, the high reliable organic transistor can beprovided.

Embodiment Mode 4

In this embodiment mode, an example of a structure of an N-type organictransistor in which electrons serve as carriers is explained using FIG.5A and 5B. The source and drain electrodes 16 a and 16 b in theembodiment mode 1 have a second composite layer 58 including alkalimetal, alkaline-earth metal, or a compound including them (an oxide, anitride, or salt) additionally. In this embodiment mode, the compositelayer 14 having the organic oxide and the metal oxide of Embodiment Mode1 is referred to as a first composite layer 14. Elements similar tothose of Embodiment Mode 1 are denoted by the same reference numerals,and the description is omitted.

The semiconductor material which is written in Embodiment Mode 1 can beused as the organic semiconductor material used for a semiconductorlayer 53. In particular, a material which is cited as an N-typesemiconductor is preferable.

The type of alkali metal, alkaline-earth metal, or a compound includingthem (an oxide, a nitride, or salt) is not limited particularly.However, lithium, sodium, potassium, cesium, magnesium, calcium,strontium, barium, lithium oxide, magnesium nitride, or calcium nitridewhich is to be given below is preferable. Note that a method formanufacturing the second composite layer 58 is not limited, as long asthe semiconductor layer 53 does not resolved, and a sputtering method oran evaporation method can be used.

In addition, the second composite layer 58 may be formed by a mixedmaterial of these materials and the organic compound having an electrontransport property. As the organic material having an electron transportproperty, there is perylene tetra carboxylic anhydride and thederivative thereof, a perylene tetra carboxylic diimide derivative,naphthalene tetra carboxylic anhydride and the derivative, a naphthalenetetra carboxylic diimide derivative, a metallophthalocyanine derivative,or fullerene series. Additionally, for example, the material which iscomposed of metallic complex having a quinoline skeleton or abenzoquinoline skeleton such as tris(8-quinolinolato)aluminum(abbreviated to ANA tris(4-methyl-8-quinolinolato)aluminum (abbreviatedto Almq₃), bis(10-hydroxybenz)[h]-quinolinato)beryllium (abbreviated toBeBq₂), bis (2-methyl-8-quinolinolato)-4-phenylphenolato-aluminum(abbreviated to BAlq) can be used. In addition, a metal complex havingan oxazole or thiazole ligand such as bis[2-(2-hydroxyphenyl)benzooxazolato] zinc (abbreviated to Zn(BOX)₂), bis[2-(2-hydroxyphenyl)benzothiazolato] zinc (abbreviated to Zn(BTZ)₂) canbe used. In addition to such metal complex,2-(4-biphenylyl)-5-(4-tert-butylphenyl)-1,3,4-oxadiazole (abbreviated toPBD), 1,3-bis[5-(p-tert-butylphenyl)-1,3,4-oxadiazole-2-yl]benzene(abbreviated to OXD-7),3-(4-tert-butylphenyl)-4-phenyl-5-(4-biphenylyl)-1,2,4-triazole(abbreviated to TAZ),3-(4-tert-butylphenyl)-4-(4-ethylphenyl)-5-(4-biphenylyl)-1,2,4-triazole(abbreviated to p-EtTAZ), bathophenanthroline (abbreviated to BPhen),bathocuproin (abbreviated to BCP), or the like can be used. Note thatthe second composite layer 58 in which the organic compound having anelectron transporting property is mixed may be formed by aco-evaporation method using resistance heating; a co-evaporation methodusing resistance heating and an electron gun evaporation (EBevaporation); simultaneous deposition using sputtering and resistanceheating, or the like.

First, the conductive layer 17 which becomes a gate electrode is formedover a substrate 10. Next, a gate insulating film 12 which made of thenitride, oxide, or oxynitride of the gate electrode material is formedby nitriding or oxygenizing the conductive layer 17 which becomes thegate electrode by using dense plasma. Furthermore, the source and drainelectrodes 56 a and 56 b having the second composite layer 58, the firstcomposite layer 14, and the conductive layer 15 are formed, thus theorganic transistor is manufactured. An inert gas used for a gas to besupplied such as argon, krypton, helium, or xenon as shown in EmbodimentMode 1 is mixed in the gate insulating film 12.

In the organic transistor manufactured by the above method, when avoltage is applied to an electrode having a structure in which thesecond composite layer 58, the first composite layer 14, and theconductive layer 15 are stacked, holes and electrons are generated bycarrier separating in vicinity of the boundary face of the secondcomposite layer 58 and the first composite layer 14. The electrons ofthe generated carriers are supplied to the semiconductor layer 53 fromthe second composite layer 58, and the holes of the generated carriersflow to the conductive layer 15. In this manner, a current in which theelectron is carrier flows into the semiconductor layer 53.

In addition, an energy barrier between the semiconductor layer 53 andthe source and drain electrodes 56 a and 56 b is reduced by using thesource and drain electrodes 56 a and 56 b which have a structure inwhich the composite layer 14 is interposed between the semiconductorlayer 53 and the conductive layer 15, and carrier injection from one ofthe electrode of the source and drain electrodes to the semiconductorlayer and carrier discharging from the semiconductor layer to the otherelectrode become easily. Consequently, a material of the conductivelayer 15 does not have to select the material in which the energybarrier is low, and can be selected without a constraint of a workfunction.

In addition, the first composite layer 14 is superior in a carrierinjection property and chemically stable and adhesion with thesemiconductor 53 is better than the conductive layer 15. The source anddrain electrodes 56 a and 56 b of the present invention can be used alsoas a wiring.

Since the gate insulating film formed by using dense plasma is fewplasma damage and defects, a tunnel leakage current can be reduced.Since asperity of the surface is small, carrier mobility can be high.Further, it makes orientation of the organic semiconductor materialwhich makes the semiconductor layer 53 formed over the gate insulatingfilm easy. In addition, high dielectric constant gate insulating filmcan be formed by selecting a material such as Ta or Al to the gateelectrode which becomes high dielectric constant by a nitridingtreatment or an oxidation treatment. Therefore, even if the gateinsulating film is made a thinner, a physical film thickness can begained, and high speed operation can be performed while preventing atunnel leakage current. Further, since a width of the gate electrode canbe narrowed and the gate electrode can be made thinner by reacting thegate electrode directly, a channel-length can be shortened. Therefore,high integration is enabled.

An organic insulating material such as polyimide, polyamic acid, orpolyvinylphenyl may be formed a film in contact with a under surface ofthe semiconductor layer 53. With such a structure, orientation of theorganic semiconductor material is more improved, and adhesion of thegate insulating film 12 and the semiconductor layer 53 can be moreimproved.

As described above, the high reliable organic transistor can beprovided.

Embodiment Mode 5

A mode of a liquid crystal display device (liquid crystal device)including an organic transistor of the present invention is explainedusing FIG. 7.

FIG. 7 is a schematic top view showing a liquid crystal display device.Reference numeral 601, which is indicated by a dotted line, is a drivercircuit portion (a source side driver circuit), 602 is a pixel portion,603 is a driver circuit portion (a gate side driver circuit), and theseare sealed by an element substrate 600, a counter substrate 604, and asealant 605.

The source side driver circuit 601 and the gate side driver circuit 603receive a video signal, a clock signal, a start signal, a reset signaland the like from an FPC (flexible printed circuit) 609 serving as anexternal input terminal. Though the only FPC is illustrated here, aprinted wiring board (PWB) may be mounted on the FPC.

The liquid crystal display device in the present invention includes notonly the liquid crystal display device itself but also a state where anFPC or a PWB is mounted thereto.

The pixel portion 602 is not limited especially, for example, the pixelportion 602 has a liquid crystal display element (liquid crystalelement) and a transistor for driving the liquid crystal display elementas shown in a sectional-view of FIG. 8.

The liquid crystal display device shown in the sectional-view of FIG. 8has an organic transistor 527 having the gate insulating film 12 formedby insulating the conductive layer which becomes the gate electrode bydense plasma treatment same as the organic transistor written inEmbodiment Mode 1. Portions similar to those of Embodiment Mode 1 aredenoted by the same reference numerals and the description thereof isomitted.

The organic transistor 527 formed over the substrate 10 is covered withan insulating film 528. One side of the conductive layer 15 whichfunctions as a portion of the source and drain electrodes iselectrically connected to a pixel electrode 529 through a contact hole.The liquid crystal display element is formed by sandwiching a liquidcrystal layer 534 between a counter electrode 532 formed over a countersubstrate 531 and the pixel electrode 529. Orientation films 533 and 530are formed respectively on the surfaces of the counter electrode 532which is contact with the liquid crystal layer 534 and the pixelelectrode 529. Note that the liquid crystal layer 534 keeps cell gap bya spacer 535. The structure of the liquid crystal display device is notlimited particularly.

A light emitting device using the organic transistor 527 of the presentinvention is explained with reference to FIG. 9. The light emittingdevice shown in a sectional-view of FIG. 9 has an organic transistor 527having the gate insulating film 12 formed by insulating the conductivelayer which becomes the gate electrode by the dense plasma treatment,similarly to the organic transistor written in Embodiment Mode 1.Portions similar to those of Embodiment Mode 1 are denoted by the samereference numerals and the description thereof is omitted.

The organic transistor 527 formed over the substrate 10 is covered withthe insulating layer 528. One side of the conductive layer 15 whichfunctions as a portion of the source and drain electrodes iselectrically connected to a first electrode 610 through a contact hole.An end portion of the first electrode 610 is covered with an insulatinglayer 611, and a light emitting layer 612 is formed so as to cover aportion exposing from the insulating layer 611. A second electrode 613and a passivation film 614 are formed over the light emitting layer 612.Note that the light emitting layer 612 is isolated from outside air bysealing the substrate 10 and a counter substrate 615 by using thesealant (not illustrated) in the outside of the pixel portion. Aninterspace 616 between the counter substrate 615 and the substrate 10may be filled with an inert gas such as dried nitrogen, or the sealingmay be performed by filling the interspace 616 with resin instead of thesealant. The structure of the light emitting device is not limitedparticularly.

When the gate insulating film formed by using dense plasma is used asthe organic transistor 527, the tunnel leakage current can be reducedand a high reliable display device can be obtained.

In this embodiment mode, the insulating film formed by performing denseplasma treatment to the conductive layer which becomes the gateelectrode can be used as the gate insulating film using for the organictransistor 527. However, the gate insulating film processed by denseplasma treatment may be used.

The display device as described above can be used as a display devicemounted to an electronics device such as a telephone set or a televisionset as shown in FIGS. 10A and 10B. In addition, the display device maybe mounted to a card having a function of managing private informationsuch as an ID card as shown in FIG. 10C, to a flexible electronic paperas shown in FIG. 10D, or the like.

FIG. 10A is a diagram of a telephone set, and a main body 710 of thetelephone set includes a display portion 711, an audio output portion713, an audio input portion 714, operation switches 715 and 716, anantenna 717, and the like.

An active matrix type display device is provided in the display portion711. A display system is may be a liquid crystal display or EL display.The display portion 711 has an organic transistor in every pixel. Thegate insulating film processed by dense plasma treatment by using theabove embodiment mode is used to the organic transistor. In addition, anintegrated circuit for driving the display portion 711 is formed overthe substrate same as the organic transistor or mounted. The organictransistor having the gate insulating film processed by dense plasmatreatment by using the above embodiment mode may be used for thetransistor provided in the integrated circuit. A telephone set having agood electrical property and high reliability can be obtained by usingthe organic transistor of the present invention.

FIG. 10B shows a television set manufactured by applying the presentinvention. The television set includes a display portion 720, a housing721, a speaker 722, and the like.

An active matrix type display device is provided in the display portion720. A display system is may be a liquid crystal display or EL display.The display portion 720 has an organic transistor in every pixel. Thegate insulating film processed by dense plasma treatment by using theabove embodiment mode is used to the organic transistor. In addition, anintegrated circuit for driving the display portion 720 is formed overthe substrate same as the organic transistor or mounted. The organictransistor having the gate insulating film processed by dense plasmatreatment by using the above embodiment mode may be used to thetransistor provided in the integrated circuit. A television set having agood electrical property and high reliability can be obtained by usingthe organic transistor of the present invention.

FIG. 10C shows an ID card manufactured by applying the presentinvention. The ID card includes a support medium 730, a display portion731, an integrated circuit chip 732 incorporated in the support medium730 or the like. Integrated circuits 733 and 734 for driving the displayportion 731 are incorporated into the support medium 730.

An active matrix type display device is provided in the display portion731. A display system is may be a liquid crystal display or EL display.The display portion 731 has an organic transistor in every pixel. Thegate insulating film processed by dense plasma treatment by using theabove embodiment mode is used to the organic transistor. In addition,integrated circuits 733 and 734 for driving the display portion 731 areformed over the substrate same as the organic transistor or mounted. Theorganic transistor having the gate insulating film processed by denseplasma treatment by using the above embodiment mode may be used for thetransistor provided in the integrated circuits 733 and 734. An ID cardhaving a good electrical property and high reliability can be obtainedby using the organic transistor of the present invention.

The type of information input or output can be confirmed by displayingthe information input or output by an integrated circuit chip 732, inthe display portion 731.

FIG. 10D shows an electronic paper manufactured by applying the presentinvention. A main body 740 includes a display portion 741, a receivingapparatus 742, a driving circuit 743, a film battery 744, or the like.

An active matrix type display device is provided in the display portion741. A display method is may be a liquid crystal display or EL display.The display portion 741 has an organic transistor in every pixel. Thegate insulating film processed by dense plasma treatment by using theabove embodiment mode is used to the organic transistor. In addition, adriving circuit 743 for driving a receiving apparatus 742 and thedisplay portion 741 is formed over the substrate same as the organictransistor or mounted. The organic transistor having the gate insulatingfilm processed by dense plasma treatment by using the above embodimentmode may be used for the transistor provided in the receiving apparatus742 and the driving circuit 743. In an information processing function,a loading of the display device can be reduced by using another devicehaving the receiving apparatus 742 and a communication function. Sincethe organic transistor of the present invention can be manufactured on aflexible substrate such as a plastic substrate, it is very effective toapply the organic transistor of the present invention to an electronicpaper, and the electronic paper having a good electrical property andhigh reliability can be manufactured.

As set forth above, the application range of the invention is so widethat it can be applied to display devices in various fields. Thisembodiment mode can be freely combined with the structure of theEmbodiment Modes 1 to 4.

Embodiment 1

The organic transistor manufactured by using the present invention isexplained using FIG. 1A and 1B.

A conductive layer 17 which has a 100 nm film thickness and made ofaluminum is formed over the substrate 10 by a sputtering method. Next, agate insulating film 12 which has a 30 nm film thickness is formed byperforming dense plasma oxygenizing to the conductive layer 17. When thedense plasma treatment device in FIG. 6 is used, an interval from aplasma source to the substrate which is a processing object may be setin a range of 20 mm to 80 mm; however, a range of 20 mm. to 60 mm ispreferable. Except for the gate insulating film 12 of the conductivelayer 17 insulated by using dense plasma functions as the gate electrode11.

Next, the semiconductor layer 13 is formed over the gate insulating film12 by forming a 50 nm thick pentacene as a film so as to cover anoverlapping portion of the gate insulating film 12 and the gateelectrode 11.

A molybdenum oxide (VI) and TPD which is an aromatic amine compound areformed as a film with a 10 nm film thickness so that molar ratio is 1:1by the co-evaporation method as the composite layer 14. Furthermore,aluminum is formed by a vacuum evaporation using a mask as theconductive layer 15, and the source and drain electrodes 16 a and 16 bare manufactured.

According to the above, a high reliable P-channel organic transistor canbe obtained.

The application is based on Japanese Patent Application serial No.2005-125930 filed in Japan Patent Office on Apr. 25, 2005 the entirecontents of which are hereby incorporated by reference.

1. A method for manufacturing an organic transistor, the methodcomprising: forming a first conductive layer over a substrate;performing plasma treatment on a surface of the first conductive layerto form a gate insulating film, wherein the plasma treatment isperformed with density of electron of 10¹¹ cm⁻³ or more and an electrontemperature in a range of 0.2 eV to 2.0 eV; forming an organicsemiconductor layer over the gate insulating film; forming a compositelayer over the organic semiconductor layer, wherein the composite layercomprises an organic compound and a metal oxide which are mixed witheach other; and forming a second conductive layer over the compositelayer.
 2. The method according to claim 1, wherein the electrontemperature is in a range of 0.5 eV to 1.5 eV.
 3. The method accordingto claim 1, wherein the first conductive layer comprises one oftantalum, niobium, aluminum, molybdenum, titanium, or copper.
 4. Themethod according to claim 1, wherein the gate insulating film comprisesan oxide of a metal which is selected from tantalum, niobium, aluminum,molybdenum, titanium, and copper.
 5. The method according to claim 1,wherein the gate insulating film comprises a nitride of a metal which isselected from tantalum, niobium, aluminum, molybdenum, titanium, andcopper.
 6. The method according to claim 1, wherein the organic compoundcomprises an aromatic amine skeleton.
 7. The method according to claim1, wherein the organic compound is an aromatic hydrocarbon.
 8. Themethod according to claim 1, wherein the metal oxide is one or aplurality of oxides of titanium, vanadium, chromium, zirconium, niobium,molybdenum, hafnium, tantalum, tungsten, and rhenium.
 9. A method formanufacturing an organic transistor, the method comprising: forming agate electrode over a substrate; forming a gate insulating film over thegate electrode; performing plasma treatment on the gate insulating film,wherein the plasma treatment is performed with density of electron of10¹¹ cm⁻³ or more and an electron temperature in a range of 0.2 eV to2.0 eV; forming an organic semiconductor layer over the gate insulatingfilm after the plasma treatment, forming a composite layer over theorganic semiconductor layer, wherein the composite layer comprises anorganic compound and a metal oxide which are mixed with each other; andforming a conductive layer over the composite layer.
 10. The methodaccording to claim 9, wherein the electron temperature is in a range of0.5 eV to 1.5 eV.
 11. The method according to claim 9, wherein the gateelectrode comprises one of tantalum, niobium, aluminum, molybdenum,titanium, or copper.
 12. The method according to claim 9, wherein thegate insulating film comprises an oxide of a metal which is selectedfrom tantalum, niobium, aluminum, molybdenum, titanium, and copper. 13.The method according to claim 9, wherein the gate insulating filmcomprises a nitride of a metal which is selected from tantalum, niobium,aluminum, molybdenum, titanium, and copper.
 14. The method according toclaim 9, wherein the organic compound comprises an aromatic amineskeleton.
 15. The method according to claim 9, wherein the organiccompound is an aromatic hydrocarbon.
 16. The method according to claim9, wherein the metal oxide is one or a plurality of oxides of titanium,vanadium, chromium, zirconium, niobium, molybdenum, hafnium, tantalum,tungsten, and rhenium.
 17. A method for manufacturing an organictransistor, the method comprising: forming a first conductive layer overa substrate; performing plasma treatment on a surface of the firstconductive layer to form a gate insulating film, wherein the plasmatreatment is performed with density of electron of 10¹¹ cm⁻³ or more andan electron temperature in a range of 0.2 eV to 2.0 eV; forming anorganic semiconductor layer over the gate insulating film; forming asecond composite layer over the organic semiconductor layer, wherein thesecond composite layer comprise a second organic compound and a metalselected from an alkali metal and a alkaline-earth metal, forming afirst composite layer over the second composite layer, wherein the firstcomposite layer comprises a first organic compound and a metal oxidewhich are mixed with each other; and forming a second conductive layerover the first composite layer.
 18. The method according to claim 17,wherein the electron temperature is in a range of 0.5 eV to 1.5 eV. 19.The method according to claim 17, wherein the first conductive layercomprises one of tantalum, niobium, aluminum, molybdenum, titanium, orcopper.
 20. The method according to claim 17, wherein the gateinsulating film comprises an oxide of a metal which is selected fromtantalum, niobium, aluminum, molybdenum, titanium, and copper.
 21. Themethod according to claim 17, wherein the gate insulating film comprisesa nitride of a metal which is selected from tantalum, niobium, aluminum,molybdenum, titanium, and copper.
 22. The method according to claim 17,wherein the second organic compound comprises an aromatic amineskeleton.
 23. The method according to claim 17, wherein the organiccompound is an aromatic hydrocarbon.
 24. The method according to claim17, wherein the metal oxide is one or a plurality of oxides of titanium,vanadium, chromium, zirconium, niobium, molybdenum, hafnium, tantalum,tungsten, and rhenium.
 25. The method according to claim 17, wherein thefirst organic compound has an electron transport property.